The present invention relates to a treatment of a reaction mixture of sucrose and a fatty acid alkyl ester. More particularly, the present invention relates to an industrially useful process for isolating purified sucrose fatty acid esters from the reaction mixture obtained in the synthesis of sucrose fatty acid esters in an organic solvent medium.
Sucrose fatty acid esters (sugar esters) useful as surface active agents are prepared industrially at present by either a solvent process wherein sucrose is reacted with a methyl ester of a higher fatty acid having 8 to 22 carbon atoms in the presence of a suitable catalyst in an organic solvent such as dimethylformamide or dimethylsulfoxide, as disclosed in Japanese Patent Publication Kokoku No. 35-13102; or an aqueous medium process wherein sucrose is formed into a molten mixture with a fatty acid salt (soap) using water without using an organic solvent, and is then reacted with a higher fatty acid methyl ester in the presence of a catalyst, as disclosed in Japanese Patent Publication Kokoku No. 51-14485.
However, even according to any of these processes, the obtained reaction mixture contains impurities such as the unreacted sucrose, the unreacted fatty acid methyl ester, residual catalyst, soap, free fatty acid, volatile material, etc. in addition to the desired sucrose fatty acid ester. These impurities, at least impurities whose contents exceed the specified amounts must be removed prior to the sucrose fatty acid ester being put on the market. Particularly, in case of sucrose fatty acid esters used as a food additive which requires a high purity, removal of the solvent (volatile material) remaining in the product is very important in view of strict regulation, e.g. provision by FDA, U.S.A. according to which allowable content of remaining dimethylsulfoxide in sucrose fatty acid esters is at most 2 p.p.m. [Fed. Regist., 51(214), 40160-1].
In general, the conversion of sucrose is low. For example, in case of the process using dimethylformamide as the reaction medium, the conversion is at most 50%. Accordingly, recovery of the unreacted sucrose is also important.
In order to remove the remaining organic solvents and to recover the unreacted sucrose from the reaction mixture (namely crude sucrose fatty acid esters), various processes for the purification of crude product have hitherto been proposed. These purification processes usually require a large amount of organic solvents. However, in the production of sucrose fatty acid esters on an industrial scale, the use of a large amount of organic solvents has the following disadvantages: (1) risk of explosion and fire, (2) provision of explosion and fire prevention means to electric devices, (3) application of closed system to production equipment for explosion and fire prevention, (4) requirement of fireproof construction for entire building by way of precaution against explosion and fire, (5) rise in fixed cost due to the items (2), (3) and (4), (6) rise in materials cost due to loss of solvent, (7) contamination of the product with remaining solvent, and (8) adverse influence on health of workers, and increase of cost resulting from increase in labor required for the prevention therefor.
In view of these circumstances, it has been desired to develop a purification technique capable of removing the unreacted sucrose and other impurities from the crude reaction mixture without using organic solvents.
Thus, purification processes using no organic solvent have been proposed. For example, as representative methods, there have been known (1) a method wherein a sucrose fatty acid ester is precipitated by addition of an acidic aqueous solution to the reaction mixture, as disclosed in British Patent No. 809,815 and (2) a method wherein a sucrose fatty acid ester is precipitated by addition of an aqueous solution of a common neutral salt to the reaction mixture, as disclosed in Japanese Patent Publication Tokkyo Kokoku No. 42-8850.
However, these methods have disadvantages. When an acidic aqueous solution, for example, hydrochloric acid, is added to the reaction mixture as in the method (1), the sucrose fatty acid ester immediately deposits, but the unreacted sucrose is easily decomposed and converted into glucose and fruit sugar. This cannot be avoided even if the addition is conducted at a low temperature (e.g. 0.degree. to 5.degree. C.). Accordingly, the recovery and reuse of the unreacted sucrose becomes difficult.
The addition of an aqueous solution of a neutral salt such as sodium chloride or Glauber's salt, as in the method (2), causes sucrose fatty acid esters to deposit rapidly. In this case, decomposition of unreacted sucrose does not occur, but the monoester which is an effective component in the product is dissolved in the aqueous phase. Consequently, not only does the dissolution results in a large loss of the product, but also it is a hindrance particularly to production of sucrose fatty acid esters having a high HLB which are recently in great demand.
Also, in Japanese Patent Publication Tokkyo Kokai No. 51-29417, it is proposed to utilize the finding a mixture of water and a solvent used for purification (hereinafter referred to as "purification solvent") separates into an upper light layer and a lower heavy layer. Generally, the lower layer contains a large amount of water and, therefore, the unreacted sucrose, which is hydrophilic, and the salt derived from a catalyst used in the synthesis of sucrose fatty acid esters are dissolved in the lower layer. Since the upper layer contains the purification solvent in a large quantity, compounds having a small polarity such as sucrose fatty acid esters, fatty acids and unreacted fatty acid methyl esters are dissolved in the upper layer. On the other hand, the solvent used for the reaction such as dimethylsulfoxide is dissolved not only in the lower layer, but also inconveniently dissolved in the upper layer. Consequently, it is impossible to completely separate the reaction solvent only by this method. The product is contaminated with a trace amount of the reaction solvent, and the removal of such a trace amount of the solvent further requires a large amount of purification solvent.
In order to industrially realize the purification of crude sucrose fatty acid esters using water, it is very important that the reaction solvent and the purification solvent are completely removed and moreover sucrose, and the product are not lost.
Other important problems which must also be taken into consideration are recovery of the unreacted sucrose and drying of wet product, incident to the use of water as a purification solvent.
Since the purification of the reaction mixture with the use of water is based on the difference in water solubility between a sucrose fatty acid ester and unreacted sucrose, migration of a large amount of unreacted sucrose into an aqueous phase cannot be avoided. The manufacture of sucrose fatty acid esters cannot be industrially accepted unless the dissolved sucrose is recovered. Accordingly, it is also very important to efficiently recover the sucrose which has migrated into an aqueous phase upon purification.
The water-containing sucrose fatty acid ester which has been separated from the reaction mixture to be dried, is usually in the form of an aqueous solution when the water content is over 80% by weight, and in the form of a slurry when the water content is less than 80% by weight. In general, an aqueous solution of a sucrose fatty acid ester shows peculiar viscosity behavior such that the viscosity rapidly increases from about 40.degree. C., reaches maximum at about 50.degree. C. and rapidly drops over 50.degree. C. Some problems are encountered in removing the water from the sucrose fatty acid ester in the form of an aqueous solution or slurry. The evaporation of water by heating under a vacuum, for example, using a usual agitated vacuum dryer, is practically difficult because of marked foaming. Moreover, when the evaporation is conducted at a high temperature and the contacting time with the heating source is long, not only is the sucrose fatty acid ester decomposed, resulting in marked coloration or caramel formation, but also the acid value is raised by free fatty acid formed by decomposition, as disclosed in Japanese Patent Publication Tokkyo Kokoku No. 37-9966. In particular, due to the property of sucrose fatty acid esters that the softing point or melting point is low (for example, sucrose monostearate having a melting point of about 52.degree. C., and sucrose distearate having a melting point of about 110.degree. C.), the sucrose fatty acid ester itself tends to be hydrated at the final stage of evaporation of the water. This makes the dehydration more difficult. In addition, dehydration is difficult because the latent heat of evaporation of water is very high (more than 500 kcal/kg H.sub.2 O) and the evaporation temperature is high.
Other usual drying methods are also not suitable for preparing dry sucrose fatty acid esters. For example, in the case of using a flash dryer wherein a slurry is continuously heated, fed to a vacuum chamber and released thereto, various difficulties are encountered when a sufficient drying is desired because of the large latent heat of water. Even if these difficulties are overcome, the sucrose ester dehydrated and dried under a vacuum is in the molten state and, therefore, it requires a pulverization step after taking it out of the drier and cooling to less than the melting point to solidify, for instance, by blowing cold air. In addition to the requirement of many steps, there is a risk of dust explosion in the final pulverization step.
Accordingly, it is also important to solve the problems encounted by drying in the purification of sucrose fatty acid esters when using water as the purification solvent.
It is a primary object of the present invention to provide a process for obtaining a purified sucrose fatty acid ester from the reaction mixture without using organic solvents as the purification solvent.
A further object of the invention is to provide a process for removing a remaining organic solvent used as a reaction medium in the synthesis of sucrose fatty acid esters from the crude product.
A still further object of the invention is to provide a process for purifying the crude product of sucrose fatty acid esters without using an organic solvent as the purification solvent and without substantial loss of the sucrose fatty acid esters.
Another object of the invention is to provide a process for easily preparing a dry powder of a sucrose fatty acid ester having an improved purity and without deteriorating the quality in the drying step, while recovering the unreacted sucrose.
These and other objects of the present invention will become apparent from the description hereinafter.